don - that was my initial intuition on this, but the site is responsive and modal windows don't really translate well to mobile sites. I've actually figured this out, though. The solution is below:
Within my custom Quip comment chunk, I've included a QuipReply call that looks like this (below the comment content):
[[!QuipReply? &thread=`[[+thread]]` &tplAddComment=`quipAddCommentCustom2` &parent=`[[+id]]` ]]
I hide this form initially using the awesome and super lightweight Jquery Collapse (
http://webcloud.se/jQuery-Collapse/), and it appears upon clicking reply for each individual comment.
This solution loads quickly even with a larger number of comments, and is a decent solution for what I need. If anyone has any better solutions, please let me know!